1. Field of the Invention
The present invention relates to a latch-up restoration circuit for restoring a latch-up phenomenon generated in an electronic circuit.
2. Description of Background Information
In an electronic circuit having input and output terminals such as a microcomputer, especially in an electronic circuit made up of semiconductor devices having the CMOS (Complementary Metal-Oxide-Semiconductor) structure, it is known that an abnormal phenomenon called "latch-up" is generated when a noise due to the static electricity are applied on the input or output terminal thereof. This abnormal phenomenon called "latch-up" (referred to as latch-up phenomenon hereinafter) is such a phenomenon that the output of the circuit is fixed at a certain state irrespective of the input signal, so that a large current flows through the circuit. Once the latch-up phenomenon occurs, it will continue until the power supply is shut-off, and circuit elements can be destroyed by the excess current. For instance, in the case of a car stereo system equipped with a microcomputer to which a power current is directly supplied from the battery without flowing through a power switch so that the data is held and the reception of various instructions is enabled also during the power current is shut off, the latch-up phenomenon can not be removed by the on-off operations of the power switch in the event that the latch-up phenomenon occurs in the microcomputer. Furthermore, this problem is serious since it may result in the destruction of circuit elements if the malfunction is continued for a long time.
As a solution of this problem, it is conceivable, as illustrated in FIG. 1, to employ a measure such that Zener diodes ZD are inserted between each input or output terminal of a microcomputer M and a circuit point of a reference potential, to cut-off noises applied to each input or output terminal.
This measure is considerably effective for preventing the latch-up phenomenon, because in certain cases the prevention of the latch-up phenomenon is ensured by the insertion of the Zener diodes ZD between four or five particular input/output terminals of the microcomputer M and the reference potential circuit point, depending on the type or characteristic of the microcomputer M.
However, in some types of microcomputer, the prevention of the latch-up phenomenon can not be ensured unless the Zener diodes are inserted between almost all of the input and output terminals thereof and the reference potential point. In such a case, it is necessary to use a number of Zener diodes, and it will cause inconveniences in the cost and production control.